Summary Ovarian cancer causes more deaths than any other gynecologic cancer in the US. The dismal prognosis of patients with advanced disease remains little changed in the past 30 years. New approaches are needed. In the last grant cycle, our laboratory discovered that ovarian tumor cells and ovarian cancer tumor-initiating cells (`cancer stem cells') acquire and retain substantially more iron than their non-malignant counterparts ? a phenomenon we named ?iron addiction?. This enhanced iron acquisition and retention facilitates growth of ovarian cancer. However, we found that this enhanced iron retention also makes ovarian cancer cells exquisitely susceptible to drugs that trigger ferroptosis, an iron-dependent form of cell death. Although iron is central to ferroptosis, little is known about how iron actually confers this susceptibility. In this application, we test the hypothesis that iron plays critical, novel, and previously undescribed roles in ferroptosis, and that new targets in the ferroptosis pathway that we recently discovered might lead to successful interventions in ovarian cancer. We approach this problem with two broad objectives: 1) to better understand the role of iron in ferroptosis; 2) to identify specific targets that will enhance the activity of ferroptosis inducers by fostering pro-ferroptotic pathways both in ovarian cancers themselves and in the ovarian cancer microenvironment. Our Specific Aims are directed at these goals. In Aim 1, we pursue pilot observations that ferroptosis inducers trigger a signaling network that fosters the generation of polyunsaturated lipid peroxides (the proximal `executioners' of ferroptosis). We propose that ferroptosis is propagated by both 1) transcriptional activation of iron-dependent pro-ferroptotic proteins that increase labile iron, and 2) engagement of a feed forward loop that disables the iron-dependent lipid desaturase SCD1 that we recently showed protects against ferroptosis. We will test our hypothesis using cell culture as well as murine models of ovarian cancer. In Aim 2, we use state- of-the-art NanoSIMS imaging and MALDI-MSI to probe the sites of origin of the ferroptotic death signal, co- localizing iron with the oxidized lipids that typify ferroptosis. We confirm and expand these findings using organelle-targeted iron chelators. In Aim 3, we assess how cells in the ovarian tumor microenvironment modify the response of ovarian cancers to drugs that induce ferroptosis. We focus on macrophages and fibroblasts, cells that are critically involved in ovarian cancer metastasis, which we discovered in pilot studies exert paracrine effects on lipid and iron metabolism that dramatically affect the degree of ferroptosis in ovarian cancer cells. Collectively, these experiments will enhance knowledge of ovarian cancer iron metabolism, explore regulatory pathways not previously linked to ferroptosis, and define the contribution of the tumor microenvironment to ferroptosis - efforts that will help to direct more effective use of ferroptosis inducers in ovarian cancer therapy.